JP7552145B2 - Silica sol, method for producing silica sol, polishing composition, polishing method, and method for producing semiconductor device - Google Patents

Description

The present invention relates to a silica sol, a method for producing a silica sol, a polishing composition, a polishing method, a method for producing a semiconductor wafer, and a method for producing a semiconductor device.

Polishing methods using polishing liquids are known as a method for polishing the surfaces of materials such as metals and inorganic compounds. In particular, in the final polishing of prime silicon wafers for semiconductors and reclaimed silicon wafers, as well as in chemical mechanical polishing (CMP) for planarizing interlayer insulating films during semiconductor device manufacturing, forming metal plugs, forming embedded wiring, and the like, the surface condition has a significant effect on the semiconductor characteristics, so the surfaces and end faces of these components must be polished with extremely high precision.

In such precision polishing, polishing compositions containing silica particles are used, and colloidal silica is widely used as the abrasive grains, which are the main component. Colloidal silica is known to be produced by different manufacturing methods, such as by thermal decomposition of silicon tetrachloride (fumed silica, etc.), by deionization of alkali silicate such as water glass, and by hydrolysis and condensation reactions of alkoxysilanes (commonly known as the "sol-gel method").

Many studies have been conducted on methods for producing silica sols containing silica particles. For example, Patent Documents 1 to 3 disclose silica sols produced by hydrolysis and condensation reactions of alkoxysilanes.

Japanese Patent Application Publication No. 11-60232 International Publication No. 2008/123373 International Publication No. 2019/131874

Silica sol produced by hydrolysis and condensation of alkoxysilane is mixed with various compounds to be used as a polishing composition for polishing. The pH of this polishing composition changes depending on the type and amount of the compounds mixed, and depending on the pH, silica particles may aggregate or settle, adversely affecting the dispersion state of the silica particles and further adversely affecting polishing.
Among these, strongly acidic polishing compositions promote polishing depending on the pH, but have adverse effects on polishing due to aggregation of silica particles, and therefore their polishing performance is not necessarily sufficient.

In order to solve such a problem, it is conceivable to carry out a treatment to lower the pH of the isoelectric point of the silica sol.
However, the silica sols produced by the hydrolysis and condensation reactions of alkoxysilanes disclosed in Patent Documents 1 and 2 do not undergo treatment to lower the pH of the isoelectric point of the silica sol, and therefore have the problem that when the polishing composition is made strongly acidic, the silica particles aggregate, adversely affecting polishing.
In addition, the silica sol produced by the hydrolysis and condensation reactions of alkoxysilanes disclosed in Patent Document 3 may have a lowered pH at the isoelectric point of the silica sol by heating, but the silica particles are linked in a long chain shape, which is likely to cause uneven polishing. In addition, the secondary structure of the silica particles changes significantly before and after heating, which has a problem of adversely affecting the dispersion state of the silica particles and adversely affecting polishing.

The present invention has been made in view of these problems, and an object of the present invention is to provide a silica sol that can provide a strongly acidic polishing composition that maintains the dispersed state of silica particles. Another object of the present invention is to provide a method for producing a silica sol that can provide a strongly acidic polishing composition that maintains the dispersed state of silica particles and has little change in the average primary particle diameter, average secondary particle diameter, cv value, and association ratio of silica particles before and after heating. Still another object of the present invention is to provide a polishing composition that can maintain the dispersed state of silica particles even in a strongly acidic state and has excellent polishing performance, as well as a polishing method and a method for producing a semiconductor device using the polishing composition.

Conventional silica sols, when used as polishing compositions, especially when used as strongly acidic polishing compositions, have the problem of adversely affecting the dispersion state of silica particles and adversely affecting polishing. However, as a result of extensive research, the inventors have found that the above-mentioned problems can be solved by optimizing the association ratio of silica particles and the pH of the isoelectric point of the silica sol, and have thus completed the present invention.

That is, the gist of the present invention is as follows.
[1] A silica sol comprising silica particles having an association ratio calculated by the average secondary particle diameter measured by the DLS method/average primary particle diameter measured by the BET method of 4 or less, and having an isoelectric point of pH 2.4 or less.
[2] The silica sol according to [1], having an isoelectric point of pH 2.0 or less.
[3] The silica sol according to [1] or [2], wherein the cv value of the silica particles is 10 to 50.
[4] The silica sol according to any one of [1] to [3], wherein the average secondary particle diameter of the silica particles is 10 nm to 150 nm.
[5] The silica sol according to any one of [1] to [4], wherein the average primary particle diameter of the silica particles is 5 nm to 75 nm.
[6] The silica sol according to any one of [1] to [5], wherein the content of the alkali catalyst is 10 ppm or less.
[7] The silica sol according to any one of [1] to [6], having a metal impurity content of 1 ppm or less.
[8] The silica sol according to any one of [1] to [7], wherein the silica particles are mainly composed of a tetraalkoxysilane condensate.
[9] The silica sol according to [8], wherein the tetraalkoxysilane condensate contains a tetramethoxysilane condensate.
[10] The silica sol according to any one of [1] to [9], wherein the content of the silica particles is 3% by mass to 50% by mass based on the total amount of the silica sol.
[11] The method for producing a silica sol according to any one of [1] to [10], comprising a step of heating the silica sol at 180° C. or higher for 2 hours or more.
[12] The method for producing a silica sol according to [11], further comprising a step of reducing the ammonia content of the silica sol to 10 ppm or less before the heating.
[13] A polishing composition comprising the silica sol according to any one of [1] to [10].
[14] A polishing method comprising polishing using the polishing composition according to [13].
[15] A method for producing a semiconductor device, comprising a step of polishing with the polishing composition according to [13].

The silica sol of the present invention can provide a strongly acidic polishing composition that maintains the dispersed state of silica particles. In addition, the silica sol of the present invention has little change in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating, and can provide a strongly acidic polishing composition that maintains the dispersed state of silica particles. Furthermore, the polishing composition of the present invention can maintain the dispersed state of silica particles even when it is strongly acidic, and has excellent polishing performance.

The present invention is described in detail below, but is not limited to the following embodiments and can be modified and implemented in various ways within the scope of the gist. In this specification, when the expression "~" is used, it is used as an expression including the numerical values or physical property values before and after it.

The silica sol of the present invention contains silica particles having an association ratio calculated by the average secondary particle size measured by the DLS method/average primary particle size measured by the BET method of 4 or less, and has an isoelectric point of pH 2.4 or less.

The silica sol of the present invention preferably contains silica particles and a dispersion medium.

(Silica Particles)
The average primary particle size of the silica particles according to the present invention is preferably 5 nm to 75 nm, more preferably 10 nm to 60 nm. When the average primary particle size of the silica particles is 5 nm or more, the storage stability of the silica sol is excellent. Furthermore, when the average primary particle size of the silica particles is 75 nm or less, the surface roughness and scratches of the polished object can be reduced, and the sedimentation of the silica particles can be suppressed.

The average primary particle diameter of the silica particles is measured by the BET method. Specifically, the specific surface area of the silica particles is measured using an automatic specific surface area measuring device, and the average primary particle diameter is calculated using the following formula (1).
Average primary particle diameter (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ ))
... (1)

The average primary particle size of the silica particles according to the present invention can be set within the desired range using known conditions and methods.

The average secondary particle diameter of the silica particles according to the present invention is preferably 10 nm to 150 nm, and more preferably 20 nm to 100 nm. When the average secondary particle diameter of the silica particles is 10 nm or more, the removability of particles and the like during cleaning after polishing is excellent, and the storage stability of the silica sol is excellent. When the average secondary particle diameter of the silica particles is 150 nm or less, the surface roughness and scratches of the polished object during polishing can be reduced, the removability of particles and the like during cleaning after polishing can be excellent, and sedimentation of the silica particles can be suppressed.

The average secondary particle size of silica particles is measured by the DLS method (dynamic light scattering method). Specifically, it is measured using a dynamic light scattering particle size measuring device.

The average secondary particle size of the silica particles can be set within the desired range using known conditions and methods.

The cv value of the silica particles according to the present invention is preferably 10 to 50, more preferably 15 to 40, and even more preferably 20 to 35. When the cv value of the silica particles is 10 or more, the polishing rate for the object to be polished is excellent, and the productivity of the object to be polished is excellent. Furthermore, when the cv value of the silica particles is 50 or less, the surface roughness and scratches of the object to be polished during polishing can be reduced, and the ability to remove particles and the like during cleaning after polishing is excellent.

The cv value of the silica particles is calculated using the following formula (2) by measuring the average secondary particle diameter of the silica particles using a dynamic light scattering particle diameter measuring device.
cv value=(standard deviation (nm)/average secondary particle diameter (nm))×100 (2)

The association ratio of the silica particles according to the present invention is 4 or less, preferably 1 to 4, more preferably 1.1 to 3, and even more preferably 1.2 to 2. When the association ratio of the silica particles is 4 or less, the surface roughness and scratches on the object to be polished during polishing can be reduced, and the aggregation of the silica particles can be suppressed. Furthermore, when the association ratio of the silica particles is 1 or more, the polishing rate for the object to be polished is excellent, and the productivity of the object to be polished is excellent.

The association ratio of the silica particles is calculated from the average primary particle diameter measured by the above-mentioned measuring method and the average secondary particle diameter measured by the above-mentioned measuring method, using the following formula (3).
Association ratio = average secondary particle diameter / average primary particle diameter ... (3)

Examples of the shape of the silica particles according to the present invention include spherical, chain-like, cocoon-like (also called lump-like or peanut-like), irregular shapes (for example, wart-like, bent, branched, etc.). Among these silica particle shapes, spherical shapes are preferred when it is desired to reduce the surface roughness and scratches on the object to be polished during polishing, and irregular shapes are preferred when it is desired to increase the polishing rate for the object to be polished.

The silica particles according to the present invention preferably have no pores, since they have excellent mechanical strength and storage stability.
The presence or absence of pores in the silica particles is confirmed by a multipoint BET analysis using an adsorption isotherm with nitrogen as the adsorption gas.

The silica particles according to the present invention preferably contain an alkoxysilane condensate as the main component, and more preferably a tetraalkoxysilane condensate as the main component, because they have excellent mechanical strength and storage stability. The main component means that the silica particles contain 50% by mass or more of the total components constituting the silica particles (100% by mass).
In order to obtain silica particles mainly composed of alkoxysilane condensates, it is preferable to use alkoxysilane as the main raw material. In order to obtain silica particles mainly composed of tetraalkoxysilane condensates, it is preferable to use tetraalkoxysilane as the main raw material. The main raw material means that the main raw material is 50% by mass or more out of 100% by mass of all raw materials constituting the silica particles.

(Method for producing silica particles)
As a method for producing the silica particles according to the present invention, a method in which tetraalkoxysilane is subjected to hydrolysis reaction and condensation reaction is preferred, since the metal impurity content of the silica sol can be reduced, and a method in which a solution containing tetraalkoxysilane (B) and a solution containing an alkali catalyst (C) are added to a solution containing water (A), and the tetraalkoxysilane is subjected to hydrolysis reaction and condensation reaction is preferred, since the hydrolysis reaction and condensation reaction are easy to control.

Solution (A) contains water.

It is preferable that the solution (A) contains a solvent other than water, since this provides excellent dispersibility of the tetraalkoxysilane in the reaction liquid.
Examples of the solvent other than water in the solution (A) include alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol; ketones such as acetone and methyl ethyl ketone; and esters such as ethyl acetate. These solvents may be used alone or in combination of two or more. Among these solvents, alcohols are preferred, more preferably methanol and ethanol, and even more preferably methanol, because they are easy to dissolve tetraalkoxysilane, the by-products are the same as those used in the hydrolysis reaction and the condensation reaction, and they are convenient to manufacture.

The solution (A) preferably contains an alkali catalyst, since this can increase the reaction rates of the hydrolysis reaction and condensation reaction of the tetraalkoxysilane.
Examples of the alkali catalyst in the solution (A) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide, etc. These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, ammonia is preferred because it has excellent catalytic action, is easy to control the particle shape, can suppress the inclusion of metal impurities, is highly volatile, and is easy to remove after the hydrolysis reaction and the condensation reaction.

The concentration of water in solution (A) is preferably 3% by mass to 30% by mass, and more preferably 5% by mass to 25% by mass, based on 100% by mass of solution (A). When the concentration of water in solution (A) is equal to or greater than the lower limit, it is easy to control the hydrolysis reaction rate of tetraalkoxysilane. Furthermore, when the concentration of water in solution (A) is equal to or less than the upper limit, the reaction balance between the hydrolysis reaction and the condensation reaction is good, and the particle shape is easy to control.

The concentration of the alkali catalyst in solution (A) is preferably 0.5% by mass to 2.0% by mass, and more preferably 0.6% by mass to 1.5% by mass, based on 100% by mass of solution (A). When the concentration of the alkali catalyst in solution (A) is equal to or greater than the lower limit, aggregation of silica particles is suppressed, and the dispersion stability of the silica particles in the resulting dispersion is excellent. When the concentration of the alkali catalyst in solution (A) is equal to or less than the upper limit, the reaction does not proceed excessively quickly, and reaction control is excellent.

The concentration of the solvent other than water in solution (A) is preferably the balance of water and the alkali catalyst.

The solution (B) contains a tetraalkoxysilane.
Examples of the tetraalkoxysilane in solution (B) include tetraalkoxysilanes having an alkoxy group with 1 to 12 carbon atoms, such as tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. These tetraalkoxysilanes may be used alone or in combination of two or more. Among these tetraalkoxysilanes, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred, because they undergo a fast hydrolysis reaction, are less likely to leave unreacted substances, are highly productive, and can easily produce a stable silica sol.

The raw material for the silica particles may be a raw material other than tetraalkoxysilane, such as a low condensation product of tetraalkoxysilane. However, due to its excellent reactivity, it is preferable that, out of 100% by mass of all the raw materials constituting the silica particles, tetraalkoxysilane is 50% by mass or more and raw materials other than tetraalkoxysilane are 50% by mass or less, and it is even more preferable that tetraalkoxysilane is 90% by mass or more and raw materials other than tetraalkoxysilane are 10% by mass or less.

The solution (B) may contain only tetraalkoxysilane without containing a solvent, but it is preferable that the solution (B) contains a solvent, since this improves the dispersibility of the tetraalkoxysilane in the reaction liquid.
Examples of the solvent in the solution (B) include alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol; ketones such as acetone and methyl ethyl ketone; and esters such as ethyl acetate. These solvents may be used alone or in combination of two or more. Among these solvents, alcohols are preferred, more preferably methanol and ethanol, and even more preferably methanol, because the solvents used in the hydrolysis reaction and the by-products are the same and are convenient to manufacture.

The concentration of tetraalkoxysilane in solution (B) is preferably 60% by mass to 95% by mass, and more preferably 70% by mass to 90% by mass, in 100% by mass of solution (B). When the concentration of tetraalkoxysilane in solution (B) is equal to or higher than the lower limit, the reaction solution tends to become homogeneous. Furthermore, when the concentration of tetraalkoxysilane in solution (B) is equal to or lower than the upper limit, the formation of a gel-like substance can be suppressed.

The concentration of the solvent in solution (B) is preferably 5% by mass to 40% by mass, and more preferably 10% by mass to 30% by mass, based on 100% by mass of solution (B). When the concentration of the solvent in solution (B) is equal to or higher than the lower limit, the formation of a gel-like substance can be suppressed. Furthermore, when the concentration of the solvent in solution (B) is equal to or lower than the upper limit, the reaction solution tends to become homogeneous.

The addition rate of solution (B) per hour relative to the volume of solution (A) is preferably 0.05 kg/hour/L to 1.3 kg/hour/L, and more preferably 0.1 kg/hour/L to 0.8 kg/hour/L. When the addition rate of solution (B) is equal to or higher than the lower limit, the productivity of silica particles is excellent. When the addition rate of solution (B) is equal to or lower than the upper limit, the formation of a gel-like substance can be suppressed.
Here, "kg/hour/L" represents the mass (kg) of solution (B) added per hour to 1 L of solution (A).

The solution (C) contains an alkali catalyst.
Examples of the alkali catalyst in the solution (C) include ethylenediamine, diethylenetriamine, triethylenetetraamine, ammonia, urea, ethanolamine, tetramethylammonium hydroxide, etc. These alkali catalysts may be used alone or in combination of two or more. Among these alkali catalysts, ammonia is preferred because it has excellent catalytic action, is easy to control the particle shape, can suppress the inclusion of metal impurities, is highly volatile, and is easy to remove after the hydrolysis reaction and the condensation reaction.

It is preferable that the solution (C) contains a solvent, since this can reduce fluctuations in the concentration of the alkali catalyst in the reaction liquid.
Examples of the solvent in the solution (C) include water, methanol, ethanol, propanol, isopropanol, and ethylene glycol. These solvents may be used alone or in combination of two or more. Among these solvents, water and alcohol are preferred, and water is more preferred, because the solvents used in the hydrolysis reaction and the solvents produced as by-products are the same, and therefore the convenience of production is excellent.

The concentration of the alkaline catalyst in solution (C) is preferably 0.5% by mass to 10% by mass, and more preferably 1% by mass to 6% by mass, based on 100% by mass of solution (C). When the concentration of the alkaline catalyst in solution (C) is equal to or greater than the lower limit, it is easy to adjust the concentration of the alkaline catalyst in the reaction liquid from the start of the reaction to the end of the reaction. Furthermore, when the concentration of the alkaline catalyst in solution (C) is equal to or less than the upper limit, it is possible to reduce fluctuations in the concentration of the alkaline catalyst in the reaction liquid.

The concentration of the solvent in solution (C) is preferably 90% by mass to 99.5% by mass, and more preferably 94% by mass to 99% by mass, based on 100% by mass of solution (C). When the concentration of the solvent in solution (C) is equal to or greater than the lower limit, the fluctuation in the concentration of the alkaline catalyst in the reaction liquid can be reduced. Furthermore, when the concentration of the solvent in solution (C) is equal to or less than the upper limit, the concentration of the alkaline catalyst in the reaction liquid can be easily adjusted from the start of the reaction to the end of the reaction.

The addition rate of solution (C) per hour relative to the volume of solution (A) is preferably 0.02 kg/hour/L to 0.5 kg/hour/L, and more preferably 0.04 kg/hour/L to 0.3 kg/hour/L. When the addition rate of solution (C) is equal to or higher than the lower limit, the productivity of silica particles is excellent. When the addition rate of solution (C) is equal to or lower than the upper limit, the formation of a gel-like substance can be suppressed.
Here, "kg/hour/L" represents the mass (kg) of solution (C) added per hour to 1 L of solution (A).

The addition of solutions (B) and (C) is preferably carried out into solution (A). By adding solutions (B) and (C) into solution (A), when a highly volatile alkaline catalyst such as ammonia is used and when the hydrolysis reaction and condensation reaction are to be carried out at a high reaction temperature, the mixing of the components in the reaction solution is improved, abnormal reactions in the air can be suppressed, and the particle size can be easily controlled. Adding into the liquid means adding below the liquid level. For example, by making the supply outlet for solution (B) and the supply outlet for solution (C) below the liquid level of solution (A), solutions (B) and (C) can be added into solution (A).

The timing of adding solution (B) and solution (C) may be the same or may be different, such as alternating, but it is preferable that they are the same, since this reduces fluctuations in the reaction composition and makes the operation less complicated.

The reaction temperature for the hydrolysis reaction and the condensation reaction is preferably 10°C to 80°C, and more preferably 20°C to 65°C. If the reaction temperature is equal to or higher than the lower limit, solidification of the solvent in the reaction solution can be suppressed. If the reaction temperature is equal to or lower than the upper limit, bumping and evaporation of the solvent can be suppressed, and fluctuations in the composition of the reaction solution can be reduced.

The concentration of water in the reaction system for the hydrolysis reaction and the condensation reaction is preferably maintained at 3% to 30% by mass, and more preferably at 5% to 25% by mass, based on the total amount of the reaction system (100% by mass). When the concentration of water in the reaction system is equal to or higher than the lower limit, it is easy to control the hydrolysis reaction rate of the tetraalkoxysilane. Furthermore, when the concentration of water in the reaction system is equal to or lower than the upper limit, the reaction balance between the hydrolysis reaction and the condensation reaction is good, and the particle shape is easy to control.

The concentration of the alkali catalyst in the reaction system for the hydrolysis reaction and the condensation reaction is preferably maintained at 0.5% to 2.0% by mass, and more preferably at 0.6% to 1.5% by mass, based on the total amount in the reaction system (100% by mass). When the concentration of the alkali catalyst in the reaction system is equal to or higher than the lower limit, aggregation of the silica particles is suppressed, and the dispersion stability of the silica particles in the dispersion is excellent. Furthermore, when the concentration of the alkali catalyst in the reaction system is equal to or lower than the upper limit, the reaction does not proceed excessively quickly, and reaction control is excellent.

The method for producing silica particles according to the present invention can remove unnecessary components and add necessary components, and therefore preferably further includes a step of concentrating the obtained dispersion of silica particles and adding a dispersion medium after the above hydrolysis reaction and condensation reaction steps.

The silica particle dispersion may be concentrated first, or the dispersion medium may be added at the same time.

The method for concentrating the dispersion of silica particles is not particularly limited, and examples thereof include a heat concentration method and a membrane concentration method.
In order to concentrate the dispersion of silica particles by the heat concentration method, the dispersion may be heated and concentrated under normal pressure or reduced pressure.
To concentrate a dispersion of silica particles by the membrane concentration method, membrane separation by ultrafiltration is preferred. The molecular weight cutoff of the ultrafiltration membrane used here is selected according to the particle size of the silica particles in the dispersion.
Examples of the material of the ultrafiltration membrane include polysulfone, polyacrylonitrile, sintered metal, ceramic, carbon, etc. Examples of the form of the ultrafiltration membrane include spiral type, tubular type, hollow fiber type, etc.

Examples of the dispersion medium to be added to the dispersion liquid of silica particles include water, and alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media may be used alone or in combination of two or more. Among these dispersion media, water and alcohol are preferred, and water is more preferred, because they have excellent affinity with silica particles.

(Silica sol)
The silica sol of the present invention has an isoelectric point of pH 2.4 or less. The isoelectric point of the silica sol of the present invention is preferably pH 0 to 2.2, more preferably pH 0.1 to 2.0. When the isoelectric point is pH 2.4 or less, the dispersibility of silica particles in a strongly acidic polishing composition is excellent. In addition, when the isoelectric point is pH 0 or more, the production of the silica sol becomes easy.
The pH of the isoelectric point of the silica sol is measured at 25° C. using a zeta potential measuring device.

Examples of the dispersion medium in the silica sol include water, and alcohols such as methanol, ethanol, propanol, isopropanol, and ethylene glycol. These dispersion media in the silica sol may be used alone or in combination of two or more. Among these dispersion media in the silica sol, water and alcohol are preferred, and water is more preferred, because they have excellent affinity with the silica particles.

The content of silica particles in the silica sol of the present invention is preferably 3% by mass to 50% by mass, more preferably 4% by mass to 40% by mass, and even more preferably 5% by mass to 30% by mass, based on the total amount of silica sol (100% by mass). When the content of silica particles in the silica sol is 3% by mass or more, the polishing rate for the object to be polished is excellent. Furthermore, when the content of silica particles in the silica sol is 50% by mass or less, aggregation of silica particles in the silica sol or polishing composition can be suppressed, and the storage stability of the silica sol or polishing composition is excellent.

The content of the dispersion medium in the silica sol of the present invention is preferably 50% by mass to 97% by mass, more preferably 60% by mass to 96% by mass, and even more preferably 70% by mass to 95% by mass, based on the total amount of the silica sol (100% by mass). When the content of the dispersion medium in the silica sol is 50% by mass or more, aggregation of silica particles in the silica sol or polishing composition can be suppressed, and the storage stability of the silica sol or polishing composition is excellent. Furthermore, when the content of the dispersion medium in the silica sol is 97% by mass or less, the polishing rate for the object to be polished is excellent.

The content of silica particles and dispersion medium in the silica sol can be set to the desired range by removing unnecessary components from the resulting dispersion of silica particles and adding necessary components.

In addition to the silica particles and the dispersion medium, the silica sol of the present invention may contain other components, such as an oxidizing agent, a preservative, an antifungal agent, a pH adjuster, a pH buffer, a surfactant, a chelating agent, and an antibacterial and biocide, as necessary, within the range that does not impair the performance of the silica sol.
In particular, it is preferable to include an antibacterial biocide in the silica sol, since this provides excellent storage stability of the silica sol.

Examples of antibacterial biocides include hydrogen peroxide, ammonia, quaternary ammonium hydroxide, quaternary ammonium salt, ethylenediamine, glutaraldehyde, hydrogen peroxide, methyl p-hydroxybenzoate, sodium chlorite, etc. These antibacterial biocides may be used alone or in combination of two or more. Among these antibacterial biocides, hydrogen peroxide is preferred because of its excellent affinity with silica sol.
Antimicrobial biocides also include those commonly referred to as germicides.

The content of the antibacterial biocide in the silica sol is preferably 0.0001% to 10% by mass, and more preferably 0.001% to 1% by mass, based on 100% by mass of the total amount of the silica sol. When the content of the antibacterial biocide in the silica sol is 0.0001% by mass or more, the storage stability of the silica sol is excellent. When the content of the antibacterial biocide in the silica sol is 10% by mass or less, the original performance of the silica sol is not impaired.

The pH of the silica sol of the present invention is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. When the pH of the silica sol is 6.0 or more, the dispersion stability is excellent and the aggregation of the silica particles can be suppressed. Furthermore, when the pH of the silica sol is 8.0 or less, dissolution of the silica particles is prevented and the long-term storage stability is excellent.
The pH of the silica sol can be adjusted to a desired range by removing the alkaline catalyst or adding a pH adjuster.

The metal impurity content in the silica sol of the present invention is preferably 1 ppm or less, and more preferably 0.4 ppm or less.

When polishing semiconductor devices, metal impurities adhere to and contaminate the surface of the object to be polished, adversely affecting the characteristics of the object and diffusing into the interior of the object to be polished, degrading its quality, resulting in a significant decrease in the performance of semiconductor devices manufactured using such objects.
Furthermore, when silica particles and metal impurities coexist in a silica sol, a coordinate interaction occurs between the acidic surface silanol groups and the metal impurities, changing the chemical properties (acidity, etc.) of the surface silanol groups and the three-dimensional environment of the silica particle surfaces (ease of aggregation of silica particles, etc.), thereby affecting the polishing rate.

The metal impurity content in silica sol is measured by inductively coupled plasma mass spectrometry (ICP-MS). Specifically, an accurate amount of silica sol containing 0.4 g of silica particles is weighed out, sulfuric acid and hydrofluoric acid are added, and the mixture is heated, dissolved, and evaporated. Pure water is added to the remaining sulfuric acid droplets so that the total amount is exactly 10 g to create a test solution, which is then measured using an inductively coupled plasma mass spectrometer. The target metals are sodium, potassium, iron, aluminum, calcium, magnesium, zinc, cobalt, chromium, copper, manganese, lead, titanium, silver, and nickel, and the sum of the contents of these metals is the metal impurity content.

The content of metal impurities in silica sol can be reduced to 1 ppm or less by obtaining silica particles through hydrolysis and condensation reactions using alkoxysilane as a main raw material.
In the method of deionizing alkali silicate such as water glass, sodium and the like derived from the raw material remain, so it is extremely difficult to reduce the metal impurity content in the silica sol to 1 ppm or less.

The content of an alkali catalyst such as ammonia in the silica sol of the present invention is preferably 50 ppm or less, and more preferably 10 ppm or less, since this can reduce the changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after the heating described below.

The content of the alkaline catalyst in the silica sol can be reduced to 50 ppm or less by removing the alkaline catalyst after the silica particles are produced and before the heating described below.

(Method for producing silica sol)
The method for producing a silica sol of the present invention can obtain a silica sol having an isoelectric point of pH 2.4 or less, and therefore preferably includes a step of heating the silica sol at 180° C. or more for 2 hours or more.

The silica sol before heating may be produced by using the above-mentioned dispersion liquid of silica particles as it is, or by removing unnecessary components from the components in the dispersion liquid of silica particles and adding necessary components. However, since changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles can be reduced and the dispersion stability of the silica sol is excellent, it is preferable to adjust the ammonia content of the silica sol to 50 ppm or less before heating, and more preferably 10 ppm or less.
In order to reduce the ammonia content of the silica sol to 50 ppm or less before heating the silica sol, the ammonia can be volatilized and removed by the above-mentioned heating and concentration or the like.

The heating temperature of the silica sol is preferably 180°C or higher, more preferably 180°C to 300°C, and more preferably 190°C to 250°C. When the heating temperature is 180°C or higher, a silica sol with an isoelectric point of pH 2.4 or lower can be obtained. Furthermore, when the heating temperature is 300°C or lower, the changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating can be reduced, and the dispersion stability of the silica sol is excellent.

The pressure during heating is preferably 1.0 MPa or more, more preferably 1.0 MPa to 9.4 MPa, and even more preferably 1.3 MPa to 4.3 MPa. When the pressure during heating is 1.0 MPa or more, a silica sol having an isoelectric point of pH 2.4 or less can be obtained. When the pressure during heating is 9.4 MPa or less, the changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating can be reduced, and the dispersion stability of the silica sol is excellent.
Pressurization can be achieved by heating the silica sol to a temperature equal to or higher than the boiling point of the dispersion medium in a sealed state. When the silica sol dispersed in water in a sealed state is heated to 100° C. or higher, the pressure becomes the saturated water vapor pressure at that temperature.

The heating time is preferably 2 hours or more, more preferably 2 to 10 hours, and even more preferably 4 to 8 hours. When the heating time is 2 hours or more, a silica sol having an isoelectric point of pH 2.4 or less can be obtained. Furthermore, when the heating time is 10 hours or less, the changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating can be reduced, and the dispersion stability of the silica sol is excellent.

It is more preferable to carry out heating in an aqueous dispersion, since this can minimize changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating, and this provides excellent dispersion stability of the silica sol.

The pH when heating the aqueous dispersion is preferably 6.0 to 8.0, more preferably 6.5 to 7.8. If the pH is 6.0 or higher, gelation of the silica sol can be suppressed. Furthermore, if the pH is 8.0 or lower, changes in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles can be reduced, resulting in excellent dispersion stability of the silica sol.

(Polishing composition)
The polishing composition of the present invention contains the silica sol of the present invention, and may contain known main components, if necessary.

The polishing composition of the present invention can maintain the dispersion state of silica particles even in a strongly acidic state, and therefore can be suitably used for polishing that is chemically acted on by acid. Specifically, chemical mechanical polishing of silicon oxide films is preferred, and chemical mechanical polishing of silicon dioxide films is even more preferred.

The pH of the polishing composition is preferably 2.4 or less, taking into consideration the pH of the isoelectric point of the silica sol.
The pH of the polishing composition is preferably 0 to 2.2, and more preferably 0.1 to 2.0. The pH of the polishing composition can be set to a desired range by adding an acid component.

The polishing composition of the present invention can be obtained by mixing the silica sol of the present invention and, if necessary, various other components. However, taking into consideration storage and transportation, it may be prepared at a high concentration first and then diluted with water or the like immediately before polishing.

(Polishing method)
The polishing method of the present invention is a method of polishing using the polishing composition of the present invention.
A specific example of the polishing method is a method in which the surface of the object to be polished is pressed against a polishing pad, the polishing composition of the present invention is dropped onto the polishing pad, and the surface of the object to be polished is polished.

(Method of manufacturing semiconductor devices)
The method for producing a semiconductor device of the present invention is a method comprising a step of polishing with the polishing composition of the present invention, and the specific polishing method is as described above.

(Application)
The silica sol of the present invention can be suitably used for polishing purposes, for example, polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing in a planarization step in manufacturing integrated circuits (chemical mechanical polishing), polishing synthetic quartz glass substrates used for photomasks and liquid crystals, polishing magnetic disk substrates, etc. Among them, it can be particularly suitably used for chemical mechanical polishing.

The present invention will be described in more detail below using examples, but the present invention is not limited to the description of the following examples as long as it does not deviate from the gist of the invention.

(Measurement of average primary particle size)
The silica sol obtained in each of the examples and comparative examples was dried at 150°C, and the specific surface area of the silica particles was measured using an automatic specific surface area measuring device "BELSORP-MR1" (model name, Microtrack BEL Co., Ltd.). The average primary particle size of the silica particles was calculated using the following formula (1), assuming the density to be 2.2 g/ ^cm3 .
Average primary particle diameter (nm) = 6000/(specific surface area (m ² /g) x density (g/cm ³ ))
... (1)

(Measurement of average secondary particle size and cv value)
The average secondary particle size of the silica particles of the silica sols obtained in the examples and comparative examples was measured using a dynamic light scattering particle size measurement device "Zetersizer Nano ZS" (model name, manufactured by Malvern Instruments), and the cv value was calculated using the following formula (2).
cv value=(standard deviation (nm)/average secondary particle diameter (nm))×100 (2)

(Calculation of Association Ratio)
The association ratio was calculated from the measured average primary particle size and average secondary particle size using the following formula (3).
Association ratio = average secondary particle diameter / average primary particle diameter ... (3)

(Measurement of isoelectric point)
The isoelectric pH of the silica sol obtained in each of the examples and comparative examples was measured at 25° C. using a zeta potential measuring device (model name: Zetasizer Nano ZS, manufactured by Malvern Instruments).

[Comparative Example 1]
In the solution (A) of 8.7 parts by mass of pure water, 74.6 parts by mass of methanol, and 3.6 parts by mass of 29% by mass aqueous ammonia, a solution (B) of 100 parts by mass of tetramethoxysilane and 33.4 parts by mass of methanol and a solution (C) of 37.4 parts by mass of pure water and 5.7 parts by mass of 29% by mass aqueous ammonia were added at equal speeds over 287 minutes. During the dropwise addition, the reaction solution was stirred while maintaining the temperature at 40° C. After the dropwise addition, the reaction solution was stirred for another 30 minutes while maintaining the temperature at 40° C. to obtain a dispersion of silica particles.
The obtained dispersion of silica particles was adjusted in volume by adding pure water so that the silica particle content was approximately 20% by mass, and the temperature was raised to near the boiling point of the dispersion medium to remove methanol and ammonia, thereby obtaining silica sol A having a silica particle content of approximately 20% by mass and ammonia content below the detection limit (10 ppm or less).
The evaluation results of the obtained silica sol A are shown in Table 1.

[Example 1]
The silica sol A obtained in Comparative Example 1 was placed in a sealed container and heated at 200° C. for 3 hours to obtain the silica sol of Example 1.
The evaluation results of the obtained silica sol are shown in Table 1.

[Example 2, Comparative Examples 2 to 10]
The same procedure as in Example 1 was conducted except that the heating temperature and heating time were changed as shown in Table 1, to obtain silica sols of Example 2 and Comparative Examples 2 to 10, respectively.
The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 11]
In the solution (A) of 22.5 parts by mass of pure water, 187.6 parts by mass of methanol, and 8.3 parts by mass of 29% by mass aqueous ammonia, a solution (B) of 100 parts by mass of tetramethoxysilane and 33.4 parts by mass of methanol and a solution (C) of 37.8 parts by mass of pure water and 5.2 parts by mass of 29% by mass aqueous ammonia were added at equal speeds over 109 minutes. During the dropwise addition, the reaction solution was stirred while maintaining the temperature of the reaction solution at 31 ° C. After the dropwise addition, the reaction solution was further stirred for 30 minutes while maintaining the temperature of the reaction solution at 31 ° C. to obtain a dispersion of silica particles.
The obtained dispersion of silica particles was adjusted in volume by adding pure water so that the silica particle content was approximately 20% by mass, and the temperature was raised to near the boiling point of the dispersion medium to remove methanol and ammonia, thereby obtaining Silica Sol B having a silica particle content of approximately 20% by mass and ammonia content below the detection limit (10 ppm or less).
The evaluation results of the obtained silica sol B are shown in Table 1.

[Example 3]
The silica sol B obtained in Comparative Example 11 was placed in a sealed container and heated at 200° C. for 5 hours to obtain the silica sol of Example 3.
The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Examples 12 to 13]
The same procedure as in Example 3 was conducted except that the heating temperature and heating time were changed as shown in Table 1, to obtain silica sols of Comparative Examples 12 and 13, respectively.
The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 14]
In the solution (A) of 11.6 parts by mass of pure water, 77.7 parts by mass of methanol, and 3.2 parts by mass of 29% by mass aqueous ammonia, a solution (B) of 100 parts by mass of tetramethoxysilane and 33.2 parts by mass of methanol, and a solution (C) of 31.9 parts by mass of pure water and 5.3 parts by mass of 29% by mass aqueous ammonia were added at equal speeds over 225 minutes. During the dropwise addition, the reaction solution was stirred while maintaining the temperature at 37° C. After the dropwise addition, the reaction solution was stirred for another 30 minutes while maintaining the temperature at 37° C. to obtain a dispersion of silica particles.
The obtained dispersion of silica particles was adjusted in volume by adding pure water so that the silica particle content was approximately 20% by mass, and the temperature was raised to near the boiling point of the dispersion medium to remove methanol and ammonia, thereby obtaining silica sol C with a silica particle content of approximately 20% by mass and ammonia content below the detection limit (10 ppm or less).
The evaluation results of the obtained silica sol C are shown in Table 1.

[Example 4]
The silica sol C obtained in Comparative Example 14 was placed in a sealed container and heated at 200° C. for 5 hours to obtain the silica sol of Example 4.
The evaluation results of the obtained silica sol are shown in Table 1.

[Comparative Example 15]
A commercially available silica sol (product name "PL-3", manufactured by Fuso Chemical Co., Ltd.) was used as it was as silica sol D.
The evaluation results of the silica sol D used are shown in Table 1.

[Example 5]
The silica sol D used in Comparative Example 15 was placed in a sealed container and heated at 200° C. for 5 hours to obtain the silica sol of Example 5.

[Comparative Examples 16 to 17]
The same procedure as in Example 5 was conducted except that the heating temperature and heating time were changed as shown in Table 1, to obtain silica sols of Comparative Examples 16 and 17, respectively.
The evaluation results of the obtained silica sol are shown in Table 1.

As can be seen from Table 1, it was confirmed that there was almost no change in the average primary particle size, average secondary particle size, cv value, and association ratio of the silica particles before and after heating the silica sol.
In addition, the silica sol obtained in the embodiment can lower the pH of the isoelectric point compared with the silica sol obtained in the comparative example. Therefore, it is considered that the silica sol obtained in the embodiment can obtain a strongly acidic polishing composition that maintains the dispersion state of silica particles.

The silica sol of the present invention can be suitably used for polishing purposes. For example, it can be used for polishing semiconductor materials such as silicon wafers, polishing electronic materials such as hard disk substrates, polishing in the planarization process when manufacturing integrated circuits (chemical mechanical polishing), polishing synthetic quartz glass substrates used in photomasks and liquid crystal displays, polishing magnetic disk substrates, etc. Among these, it can be particularly suitably used for chemical mechanical polishing.

Claims (16)

A silica sol comprising silica particles having an association ratio of 4 or less, calculated by the average secondary particle size measured by a DLS method/the average primary particle size measured by a BET method, and having a pH of 6.0 to 8.0 and an isoelectric point of pH 2.4 or less. The silica sol according to claim 1, having an isoelectric point of pH 2.0 or less. The silica sol according to claim 1 or 2, wherein the cv value of the silica particles is 10 to 50. The silica sol according to any one of claims 1 to 3, wherein the average secondary particle diameter of the silica particles is 10 nm to 150 nm. The silica sol according to any one of claims 1 to 4, wherein the average primary particle diameter of the silica particles is 5 nm to 75 nm. The silica sol according to any one of claims 1 to 5, wherein the content of the alkali catalyst is 10 ppm or less. The silica sol according to any one of claims 1 to 6, wherein the metal impurity content is 1 ppm or less. The silica sol according to any one of claims 1 to 7, wherein the silica particles are mainly composed of a tetraalkoxysilane condensate. The silica sol according to claim 8, wherein the tetraalkoxysilane condensate includes a tetramethoxysilane condensate. The silica sol according to any one of claims 1 to 9, wherein the content of the silica particles is 3% by mass to 50% by mass of the total amount of the silica sol. The method for producing a silica sol according to any one of claims 1 to 10, comprising a step of heating the silica sol at 180°C or higher for 2 hours or more. The method for producing a silica sol according to claim 11, further comprising a step of reducing the ammonia content of the silica sol to 10 ppm or less before the heating. The method for producing a silica sol according to claim 11, wherein the heating is carried out at a pH of 6.0 to 8.0. A polishing composition comprising the silica sol according to any one of claims 1 to 10. A polishing method comprising polishing with the polishing composition according to claim 14 . A method for producing a semiconductor device, comprising a step of polishing with the polishing composition according to claim 14 .